The present invention relates to a radiation computed tomographic imaging apparatus such as an X-ray CT (computed tomography) apparatus. More particularly, the present invention relates to a radiation computed tomography apparatus capable of acquiring volume data, such as a VCT (volume CT) apparatus or a multi-row CT apparatus, and a radiation detector for use in such an apparatus.
Known X-ray CT apparatuses include, for example, one that acquires projection data for a subject by an X-ray detector having a plurality of X-ray detector channels arranged in a two-dimensional manner. The plurality of X-ray detector channels are positioned to have their width in a direction along a predefined axis with respect to the subject. Since X-ray detector channel rows are formed over a certain width in the axis direction, the X-ray detector having X-ray detector channels arranged in a two-dimensional manner is generally referred to as a multi-row detector.
In the multi-row detector, a direction along the axis is sometimes referred to as a column direction, and a direction orthogonal to the column direction as a channel direction, for example.
In the X-ray CT apparatus comprising the multi-row detector, projection data of a cross section of the subject is collected by emitting an X-ray fan beam, which has an extent in both the column and channel directions, from a predefined focal spot at a plurality of positions around the axis to the multi-row detector.
A tomographic image of the subject is produced by a reconstruction calculation based on the projection data.
Patent Document 1 discloses an X-ray CT apparatus having an X-ray detector provided with collimators for the X-ray detector channels, which collimators are arranged in the channel direction and directed toward a focal spot of an X-ray fan beam.
In the X-ray detector described in Patent Document 1, each X-ray detector channel is disposed in the channel direction to have its detecting surface for detecting X-rays directed toward the focal spot.
[Patent Document 1] Japanese Patent Application Laid Open No. H6-22949.
In the imaging using an X-ray CT apparatus, there is a possibility that the detecting surfaces in the X-ray detector may be struck also by X-rays other than those directly impinging upon the detecting surfaces from a focal spot, i.e., for example, by scatter X-rays (scatter rays), which are caused by X-rays having their direction of travel deflected due to collision with an object, such as bone, in the subject. The scatter rays contain projection information on the subject that the X-ray detector channels receiving the scatter rays should not detect. Therefore, the scatter rays cause generation of artifacts, and image quality of the tomographic image may be degraded.
Since the X-ray CT apparatus disclosed in Patent Document 1 has the collimators in the channel direction, scatter rays can be prevented from impinging upon the detecting surfaces to some extent.
However, when the detector channels are arranged in more than one row and a fan beam is used to acquire projection data, the X-ray fan beam must have a larger width. The increase in the width of the X-ray fan beam causes X-rays to impinge upon more divergent positions in the subject, thus increasing the probability of generation of scatter rays. It is therefore difficult for a collimator merely provided at each X-ray detector channel in the channel direction, as in the X-ray CT apparatus described in Patent Document 1, to effectively prevent scatter rays from impinging upon the detecting surface. Consequently, image quality is more likely degraded.